It is known that non-living organic matter such as lacquers, paints, powder-coatings, and polymers can undergo a color change, such as yellowing, on aging.
An important example of polymers that are prone to color change are superabsorbent polymers. Superabsorbent polymers (SAPs) are well known in the art. They are commonly used in sanitary and hygienic articles, such as diapers, training pants, adult incontinence products and feminine care products to increase the absorbent capacity of such products while reducing their overall bulk. SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. Typically, SAPs are lightly crosslinked hydrophilic polymers, like partially neutralized, crosslinked polyacrylic acid.
SAPs have a tendency to degrade in color after long periods of storage. The tendency of an SAP to undergo a color transition from a bright white color to a honey brown color accelerates as storage time, temperature, and humidity increase. In temperate climates, such as the United States and Europe, the rate at which an SAP undergoes color degradation is sufficiently slow such that the SAP, or article containing the SAP, typically is consumed before a color change is observable to the naked eye.
However, in tropical and subtropical climates, such as in South America and Southeast Asia, SAP color degradation is sufficiently rapid such that a color change often occurs before the SAP, or article containing the SAP, is consumed. This problem is exacerbated when the SAPs is produced far from the tropical climate, thereby increasing the time span from SAP production to use.
Even though the change in color of the SAP does not affect SAP performance, it adversely affects consumer acceptance of articles containing the color-degraded SAPs. In particular, consumers observing a color-degraded SAP in a diaper form an opinion that the diaper contains a contaminant, is somehow soiled or faulty, or is of low quality.
It has been suggested to use sulfurous reducing agents to stabilize nonliving organic matter such as SAPs against color change and impart color stability properties to the nonliving organic matter.
WO 2004/084962 relates to a method of manufacturing color-stable superabsorbent polymer particles which involves the use of a monomer mixture containing a polymerization initiator comprising a sulfinic acid derivative, such as 2-hydroxy-2-sulfinatoacetic acid, or a salt thereof.
DE 10 2007 053 619 relates to a method for the production of water-absorbing polymer structures, wherein a reducing agent containing a sulfonate, a salt of a sulfonate, or a mixture of a sulfonate and a salt of a sulfonate is added during the production method.
US 2013/0256593 discloses a superabsorbent obtainable by polymerizing a monomer solution, drying the resulting polymer and optionally surface-crosslinking the dried polymer, wherein a sulfonic acid derivative is added to the monomer mixture or to the polymer prior to drying. The preferred sulfonic acid derivative is 2-hydroxy-2-sulfonatoacetic acid disodium salt.
WO 99/18067 discloses sulfinic acid derivatives which can be used as reducing agents which do not release formaldehyde. Specifically, WO 99/18067 discloses a zinc salt composition of 2-hydroxy-2-sulfinatoacetic acid and 2-hydroxy-2-sulfonatoacetic acid comprising 20 wt.-% of sulfinic acid and 48 wt.-% of sulfonic acid.
Whereas sulfinic acid derivatives such as 2-hydroxy-2-sulfinatoacetic acid salts have strong reducing power they are less satisfactory in long term color stabilization. Sulfonic acid derivatives such as 2-hydroxy-2-sulfonatoacetic acid salts, albeit being weaker reducing agents, are more desirable for color stabilization.
For ease of dosage the sulfurous reducing agents are preferably added to the monomer mixture or the superabsorbent polymer as a solution, in particular as an aqueous solution. Especially if the solution is applied to the dried polymer or finished superabsorbent polymer it is desirable to use highly concentrated solutions in order to minimize water input or to avoid the necessity of a subsequent drying step. Unfortunately, the commercially available 2-hydroxy-2-sulfonatoacetic acid disodium salt exhibits poor aqueous solubility. There is a need for 2-hydroxy-2-sulfonatoacetic acid derivatives having higher aqueous solubility.
In addition, the known sulfurous reducing agents result in a characteristic unpleasant odor of the treated matter. Some of the reducing agents are inherently malodorous or produce side reactions products that are malodorous. Such unpleasant odor is particularly undesirable in sanitary and hygienic articles. The unpleasant odor of sulfurous reducing agents is also undesirable for reasons of occupational safety and health protection during the production of the superabsorbent polymers or the sanitary and hygienic articles manufactured therefrom.